Heretofore, for the purpose of improving impact resistance of a styrene-based resin, polystyrene has been blended with a rubber-like polymer or styrene has been polymerized in the presence of a rubber-like polymer on a commercial scale to produce a so-called rubber-modified polystyrene resin composition in which styrene is partially graft polymerized to the rubber-like polymer and the remainder of styrene is converted into polystyrene and thus which is in the state that a graft copolymer of the rubber-like polymer/styrene and polystyrene are blended.
It is known that in the above rubber-modified polystyrene resin composition, the rubber-like polymer is usually dispersed in the styrene-based polymer in a spherical form, and the size of particle has an intimate relation with impact resistance, stiffness and gloss. That is, as the size of rubber-like polymer particle is smaller, stiffness and gloss are better. To the contrary, as the size of polymer particle is smaller, impact resistance is more decreased, and below a certain limit, an effect of improving impact resistance is lost.
In the conventional rubber-modified polystyrene resin composition, to obtain the desired impact resistance, a rubber-like polymer is dispersed in a polystyrene resin in the form of particles having a particle diameter of at least 1 .mu.m, usually in the range of 1 to 10 .mu.m. The conventional rubber-modified polystyrene resin, however, has a problem of being limited in use because it is poor in gloss and stiffness.
It is known that in high impact resistant polystyrene (HIPS), rubber-like polymer particles dispersed have occlusion structure (Die Angevandte Makromolekulare Chemie, Vol. 58/59, pp. 175-198 (1977). However, no detailed investigation has been made on the micro structure of dispersed particles. As such a styrene-based resin composition in which rubber-like polymer particles dispersed have the occlusion structure, a composition in which a styrene-butadiene block copolymer is dispersed in polystyrene in the form of particles having a particle diameter of 0.1 to 0.6 .mu.m and having the occlusion structure that more than 80% of the particles do not have a plane of symmetry has been proposed (West German Patent Laid-Open No. 3,345,377). This composition, however, is not sufficiently high in impact resistance.
The present invention is intended to overcome the defects of the conventional rubber-modified styrene-based resin composition and an object of the present invention is to provide a styrene-based resin composition which has a high impact resistance and further excellent gloss and stiffness, and which is well balanced in physical properties.
As a result of investigations to develop a styrene-based resin composition which is excellent in physical property balance such as impact resistance, gloss and stiffness, it has been found that a composition which contains a styrene-based polymer and a rubber-like polymer in a specified ratio, and in which the rubber-like polymer is dispersed in the styrene-based polymer in the form of particles having a occlusion structure having a plane of symmetry, a specified particle diameter and a specified particle diameter distribution, and a gel content and a swelling index are in specified ranges is suitable for the above object. Moreover the present inventors have discovered that the above composition exhibits particularly excellent physical properties when the thickness of the rubber-like polymer layer, the area average particle diameter of the rubber-like polymer and the volume fraction of the rubber-like polymer satisfy a specified relation. Based on the findings, the present invention has been accomplished.